Well I got the 6.4 lift pump and sump installed today and fuel pressure is much better now. Before cruising at 70 I could roll into it and drop the fp down to 48-50psi. Now it only goes down about 3 psi from where it is set at about 66-68 psi.
But WOT still drops to around 45-40psi until the torque converter locks up. Then it rises back up. Since my stock 6.0 hfcm has around 126,669 miles on it would it be a good idea just to replace the pump on it? Not the whole assembly but just the pump? And the 6.4 pump I got had around 70,xxx miles on it but it works fine. What do yall think?

Greg, 04cr450 on here, has reported that with his 6.4 pump feeding the 6.0 and stock spring (he had blue spring and removed it) he is holding steady at 60 PSI under all throttle conditions. Just throwing that out as a comparison to what another member is seeing. These pumps are normally rock solid, I would think restriction in fuel filters first and go from there.

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